Is this outside the US? At just about any US station gasoline goes through a 20cm spout and Diesel goes through a 30cm spout. Unless you have a pre unleaded fuel car (~1975 and older), you can't get the Diesel spout into a gasser filler. My 450SEL failed a smog check because the filler restrictor was missing.

Sixto
95 S420
87 300SDL